Entropy drop: Scientists create “negative temperature” system

Bizarre setup may help researchers model dark energy.

In a negative temperature system, temperatures get lower as more atoms pile up close to its maximum energy.

LMU/MPQ Munich

Over the past decades, researchers have made significant progress in cooling objects closer to absolute zero, the temperature at which all molecular motion reaches its minimum. This has allowed them to study unusual states of matter, like Bose-Einstein condensates, which behave quite differently from the materials we're familiar with. But absolute zero is as low as a temperature can get, and we can't actually reach it, so progress will ultimately be limited.

Maybe not.

As thermodynamics defines temperature, it's theoretically possible to have a negative value. Yesterday, a team of German researchers reported that they were actually able to produce a system with exactly that. They found that the negative temperature system was stable for hundreds of milliseconds, raising the prospect that we can study a radically different type of material.

To understand how temperatures can go negative, you have to think in terms of thermodynamics, which is governed by energy content and entropy. In a normal system, there's a lower limit on energy content—absolute zero—but no upper limit. If you start with a system at absolute zero and add energy, the atoms or molecules it contains start occupying higher energy states. With more energy, they start spreading out evenly among these states. This in turn increases the entropy of the system, since fewer and fewer atoms are in the same energy state.

Now imagine a system where there's an upper limit on the energy state an atom can occupy. As you add more energy, more and more atoms start occupying the maximum energy state. As this happens, entropy actually starts to go down, since an increasing fraction of the atoms begin to occupy the identical energy state. In thermodynamic terms, you've reached negative temperatures.

This has some pretty bizarre consequences. If you could maximize the entropy in the system, temperature becomes discontinuous—it jumps from positive to negative infinity. Strange things would happen if you bring it together with a system that has a normal temperature. "In thermal contact," the authors write, "heat would flow from a negative to a positive temperature system. Because negative temperature systems can absorb entropy while releasing energy, they give rise to several counterintuitive effects, such as Carnot engines with an efficiency greater than unity."

To create one of these systems, the authors set up an optical lattice of potassium atoms, chilled to near absolute zero. Under normal circumstances, these atoms repel, and thermodynamics behaves as we've come to expect. But the authors were able to switch things so that the atoms had attractive interactions. This created something that could be viewed as an "anti-pressure," which should cause the collection of atoms to collapse. It's only the negative temperature that keeps the cloud of atoms from collapsing in this set of circumstances.

It's important to emphasize that this negative temperature isn't some state "below" absolute zero. The atoms in this system still have energy, and the negative temperatures are reached through a sudden transition, rather than by gradually shifting to negative values by going past absolute zero.

Still, the system is more than just a quirky consequence of how we define temperature, since it really behaves quite differently from normal systems. The authors suggest it may help us model dark energy, in which the expansion of the Universe is a product of the sort of "negative pressure" that this system displays.

120 Reader Comments

If I recall my Statistical Mechanics and Thermodynamics courses correctly, by many definitions of temperature, a lasing system has negative temperature as well. In a system that is lasing, a large amount of the molecules/atoms/whatever, are in the same higher energy state, which then drop to the lower state coherently, emitting the laser light. Yet another example of a high energy, low entropy system.

I guess this makes a kind of sense- with some systems, things can get so disorganized that they essentially become smooth. Take those 'pin grids' that were popular in the 80s and 90s. At a certain point, so many of the pins are in the up position that the grid looks smooth again.

I think the most influential part of this study was that it behaved consistent with anti-matter, and also caused reverse reactions of the QCD and gravitational forces (it didn't cause collapse in the nucleus and repelled from other parts of the lattice).

It's been a while since I took statistical mechanics, but I seem to call that the derivation of dQ = T*dS made some assumptions that are being explicitly violated here, and so saying that this represents a negative temperature since Q and S are moving in opposite directions isn't really applicable. I'm pretty sure that if you're operating in that framework then you would also be talking about negative entropy. Negative temperature isn't that strange, in that we can make calculations based on negative energies and physical laws don't break down. But negative entropy means, what, what we have better than perfect information about the particles in the crystal? That's a logical absurdity there.

Bukkiah: The speed of an atom is limited by the speed of light, but as it approaches that speed it's mass increases without limit. So there's no limit to the kinetic energy of a particle (1/2mv^2) or its momentum (mv).

No...what we call "cold" is still "hot," thermally speaking. That is, it contains heat energy at some level greater than zero. They are both positive vectors of the same kind of energy. This is something different, in what seems to me to be a similar way that "vacuum" is different than "anti-matter."

EDIT: I'm not a physicist, this is just my intuitive grasp of the idea.

But negative entropy means, what, what we have better than perfect information about the particles in the crystal? That's a logical absurdity there.

Negative rate of entropy... is possible. There's an assumption that if you inject energy to the (large) system , then the entropy of the system should increase too. Of course, if you observe at a tiny region of the micro states of the system, you can actually see that sometimes the entropy rate is negative. This is also true for very small systems too, where classic thermodynamics and statistical mechanics are not so accurate anymore.

Temperature in physics is not a discrete property but it is a slope -- a derivative, the change in the energy of a system relative to the change in entropy. Usually, as energy goes up, entropy goes up (and vice versa) so temperature is almost always positive.

Now imagine you have a situation where particles can be in one of two states: low energy or high energy. If every particle in the system is the 'high energy' state, that gives the maximum total system energy. However, this system has the lowest possible entropy (every particle the same = no disorder, no entropy). Strangely, if you decrease the system energy -- by allowing some of those particles to randomly move to their 'low energy' states, so you end up with a mix -- entropy increases (more randomness). Because the slope here is backwards of what it should be (more energy -> less chaos, less energy --> more chaos) we say the temperature is negative.

So the strangeness is partly because we humans have chosen to define temperature.

As people have pointed out, for this to work you need a system that has a 'maximum energy', and that concept doesn't usually appear in our daily lives. I haven't been in the field for a while, but past experiments probably used atomic spin systems where because of quantum mechanics atoms could be limited, as in my example, of being in only one of a limited number of states, with clear maximum energy.

What this team did differently is they demonstrated the phenomenon with momentum. From what I understand, you can imagine they filled an energy 'bowl' with marbles (or maybe an ice cube tray, because they used a lattice) and then inverted the bowl in such a way that it did not disturb the marbles. They ended up with a bunch of marbles perched on top of this upside down bowl, this hill, in the highest possible position (energetically), but with low entropy.

Temperature in physics is not a discrete property but it is a slope -- a derivative, the change in the energy of a system relative to the change in entropy. Usually, as temperature goes up, entropy goes up (and vice versa) so temperature is almost always positive.

Please tell me I'm not the only one who looked at this article (saw it over on wired earlier today) and thought in an abstract way they've made eezo! (magnetic trap = electrical current. Just so happened they used potassium atoms near absolute zero. So, in an abstract sence, its a frozen bannana split eezo Delicious mass effect!)

Ok, but in all seriousness, what really got me is the inverse gravity effect seen. Its not antimater, its anti-entropy matter? I might be a bit hyped due to the 'cool' factor, but conceptually this could be one of the biggest breakthroughs we've seen. Who knows what new fields it will open in science if they can scale this.

Thermodynamics theory is broken because it doesn't recognize an abosolute temperature.

Consider, if the upper speed limit of matter in our universe so far observed is the speed of light. How is that you can have molecules / atoms in motion at a speed/energy higher then this?

I think you misunderstand the speed of light. You can't observe other objects moving away from you faster then c, but from a particle's point of view it can go as fast as it likes. Thus there will be no upper bound on temperature, although the need to correct for relativity will probably make computing the temperature a little more complicated.